1. Field of the Invention
This invention relates to a wavelength-conversion type light emitting device to wavelength-convert a light emitted from a light emitting element and, in particular, to a wavelength-conversion type light emitting device that is excellent in reliability, stable in brightness over a long term, and excellent in unevenness of emission color.
Further, this invention relates to a light emitting device that the light emitting element is sealed with a glass sealing material and, in particular, to a light emitting device that is excellent in mass productivity, in sealing property and deterioration resistance under a high-temperature and high-humidity environment, and in evenness of emission color.
Further, this invention relates to a method of making the above light emitting device.
2. Description of the Related Art
A light emitting device is conventionally known that uses an LED (light emitting diode) element as a light source. In recent years, such a light emitting device is in wide use for an automobile lighting apparatus, a backlight light source in LCD devices, a lamp in small electronic device etc., and the other uses are also promising.
A semiconductor light emitting device is proposed that white light is radiated by wavelength-converting a light emitted from the LED element by a phosphor (e.g., JP-A-2004-221619, [0009], [0014] and FIG. 1 thereof)
The semiconductor light emitting device in JP-A-2004-221619 comprises an LED with a lens-shaped resin sealing portion, and a transparent phosphor cover which is disposed around the resin sealing portion. The LED is a GaN-based semiconductor light emitting element which has an emission peak in 430 to 480 nm. The phosphor cover comprises a thin-film resin which has an elasticity to be in close contact with the resin sealing portion, and a phosphor which radiates a fluorescent light by being excited by light emitted from the semiconductor light emitting element.
The semiconductor light emitting device in JP-A-2004-221619 is advantageous in that a desired emission color with high brightness can be obtained by mixing a light emitted from the semiconductor light emitting element with a light wavelength-converted by the phosphor since the phosphor cover is disposed around the resin sealing portion.
However, the semiconductor light emitting device in JP-A-2004-221619 has the following problems.
(1) It is difficult to secure its long-term reliability since the resin sealing portion and the phosphor cover deteriorate due to the light emitted from the GaN-based semiconductor light emitting element. Further, the brightness of the light emitting device lowers due to the deterioration.
(2) The profile accuracy of the phosphor cover and the uniformity of phosphor dispersed must be enhanced in consideration of the unevenness in emission color and the light distribution property since the light radiation characteristics of the semiconductor light emitting device are dependent on the shaping property of the resin sealing portion and the phosphor cover. Therefore, the manufacturing process will be complicated and the manufacturing cost will increase.
On the other hand, a resin-sealed type LED is conventionally known that an LED element is sealed with a transparent resin material such as an epoxy resin.
It is known that the resin-sealed type LED is subjected to a deterioration such as yellowing when the transparent resin material is reacted with intense light while it is excellent in sealing workability due to using the transparent resin material. Especially in using a group III nitride-based compound semiconductor light emitting element to emit short-wavelength light, the transparent resin material near the element can be yellowed due to high-energy light emitted from the element and heat generated from the element. Therefore, the light extraction efficiency may lower significantly.
To prevent the deterioration of the sealing material, a light emitting device is proposed that uses a low-melting glass as the sealing material (e.g., JP-A-11-177129, [0007] and FIG. 1 thereof).
FIG. 15 is a cross sectional view showing the light emitting device disclosed in JP-A-11-177129. The light emitting device 50 comprises an LED element 51, a printed-circuit board 52, a wiring pattern 53 formed on the surface of the printed-circuit board 52, a wire 54 which electrically connects between the LED element 51 and the wiring pattern 53, and the low-melting glass 55 which seals the LED element 51 and the wire 54, and has a refractive index of about 2 which is near 2.3 or so, the refractive index of a GaN-based LED element.
The light emitting device in JP-A-11-177129 is advantageous in that a light returned to the inside of the LED element 51 due to total reflection on the surface thereof can be reduced by sealing the LED element 51 with the low-melting glass 55 which has a refractive index close to that of the GaN-based LED element. Thus, the amount of light entering into the low-melting glass 55 after being emitted from the LED element 51 can be increased. As a result, the light extraction efficiency can be enhanced as compared to the conventional device with the LED element sealed with the epoxy resin.
However, the light emitting device in JP-A-11-177129 has problems in practical manufacturing and mass productivity since the low-melting glass cannot be easy processed like the epoxy resin.
For example, when the LED element is sealed with the glass in high-viscosity state so as to prevent the heat damage of the LED element, the wire may be deformed by the high-viscosity glass so that the electrical short-circuiting or the disconnection of wire may occur. Even when using the glass in low-viscosity state, the molding as shown in FIG. 15 is difficult to conduct. On the other hand, a resin printed-circuit board cannot endure the processing temperature, and an inorganic printed-circuit board may be broken when being pressed by a mold. Further, since the glass-sealed LED element requires an individual processing, not a batch processing due to the high-temperature processing, it cannot be applied to the mass production.
As described above, a phosphor white LED with a good long-term reliability is never proposed, and a glass-sealed LED with a good mass productivity is never proposed.